1. Field of the Invention
The present invention relates to an optical disc device including CD-ROMs, CD-Rs, CD-RWs, DVDs, and Blu-ray (Trademark) Disc drives, a control method thereof, and a computer-readable recording medium.
2. Description of the Related Art
Recently, a wide variety of optical disc media have been developed and flexibly used according to intended use. In general, the optical disc media have a multi-layer structure. For example, a protective layer is formed on both sides of a medium and a data recording layer is sandwiched between those protective layers.
However, the optical disc media differ from one another in many parameters such as thickness of an optical disc medium itself, a distance between a surface of the protective layer and a surface (signal surface) of the data recording layer, the number of signal surfaces, and wavelength of a laser beam to be used for reading information from the signal surface. For example, a DVD has the signal surfaces on two layers at maximum.
Under the above-mentioned circumstances, a drive dedicated to each type of optical disc media is generally used. However, when it is necessary for a user to purchase and install the drive dedicated to each type of disc media, the user is required to master the operation of the respective drives, and an economical burden on the user is large. There has therefore been demand for a drive (optical disc device) which can be used for multiple types of media.
For a drive which can be used for multiple types of media, technology for changing a numerical aperture of a lens with a single objective lens using an optical device having a wavelength selection characteristic has been developed. The technology utilizes the fact that the wavelength of a light source (laser) used for reading differs for each type of media.
As shown in FIG. 7, an optical pickup 1 for the drive includes a light emitting element 11 for outputting laser beams having a plurality of wavelengths, a beam splitter 12, a photodetector 13, and an objective lens body 14. The objective lens body 14 includes an objective lens 14L and a hologram element 14H including a diffraction grating.
The light emitting element 11 is, for example, a semiconductor laser element (a so-called “three-wavelength laser”) which outputs laser beams having three different wavelengths. The three wavelengths are 405 nm for a Blu-ray Disc (Trademark), 650 nm for a digital versatile disc (DVD), and 780 nm for a compact disc (CD).
The beam splitter 12 guides light from the light emitting element 11 toward the objective lens body 14 side. Further, the beam splitter 12 also guides light reflected by the medium and input through the objective lens body 14 toward the photodetector 13 side. The photodetector 13 includes, for example, a plurality of light detecting elements arranged in a matrix of N×N. This photodetector 13 includes, for example, a cylindrical lens for measuring a diameter of a beam. Light guided by the beam splitter 12 reaches the plurality of light receiving elements through the cylindrical lens. Then, the photodetector 13 outputs signals corresponding to light intensities detected by the plurality of light receiving elements.
The hologram element 14H of the objective lens body 14 receives a laser beam reflected by the medium through the objective lens body 14. The hologram element 14H diffracts the received light to obtain a predetermined numerical aperture (NA) for each wavelength and guides the diffracted light to the beam splitter 12. The objective lens 14L is an aspherical lens which refracts a laser beam guided from the light emitting element 11 through the beam splitter 12 and the hologram element 14H to focus the laser beam at a position at a predetermined focal length F from the objective lens 14L, where the focal length F differs for each wavelength and output the laser beam. The objective lens 14L converges a laser beam reflected by the medium and guides it to the hologram element 14H.
This optical pickup 1 can move in a direction substantially perpendicular to the surface of the optical disc medium and the focus is controlled to be set on the signal surface of the optical disc medium before a signal is read from the optical disc medium. That is, a signal (focus error signal; FE signal) indicative of the shift of the focus of a laser beam from the recording surface of the optical disc medium, and a signal (pull-in signal; PI signal) indicative of the sum of the intensities of light reaching the light receiving elements are generated from a signal (RF signal) output from the photodetector 13. A signal (TE signal) indicative of a tracking error is usually generated from the signal output from the photodetector 13. A detailed description thereof is omitted.
The FE signal is a signal as shown in FIG. 8A. That is, the FE signal becomes substantially “0” when the signal surface is in focus. When a distance between the optical disc medium and the objective lens body 14 is changed with a focusing position as the center thereof and the objective lens body 14 moves a predetermined distance away from the focusing position, the signal has peaks on a positive side and on a negative side. This signal waveform will be referred to as a “focus error waveform” hereinafter.
The PI signal is a signal as shown in FIG. 8B. That is, this PI signal has a peak at the focusing position. FIGS. 8A and 8B are explanatory diagrams schematically showing the FE signal and the PI signal, respectively.
When the optical pickup 1 is used, the following control becomes possible. That is, a distance between the objective lens body 14 and the surface of the optical disc medium can be controlled so that a distance between a plane portion P of the objective lens 14L and the signal surface inside the medium becomes the above-mentioned focal length F, that is, so that the signal surface is brought into focus. Thus, a signal can be read from a plurality of optical disc media.
In this case, it may be judged whether the signal surface is in focus or not by using the above-mentioned FE signal and/or the PI signal. For example, it may be judged that the signal surface is in focus when the absolute value of the FE signal exceeds a peak and then falls below a predetermined threshold value (close to “0”). Alternatively, it may be judged that the signal surface is in focus when the PI signal exceeds a predetermined threshold value.
In the optical disc device using the above-mentioned optical pickup 1, before data recorded on the optical disc medium is accessed, the type of the optical disc medium is determined.
For this reason, a technique of measuring the distance from the surface of the optical disc medium to the recording surface by using a focus error signal to determine the type of optical disc medium has been developed (Japanese Patent No. 2,986,587, etc.).
However, with the diversification of optical disc media, there are types of optical disc media of which distances between the surfaces and recording surfaces are close to each other. For example, in addition to a CD and a DVD, an optical disc medium called “dual disc” which is manufactured by assembling a CD and a DVD together is available on the market.
Since a protective layer of a CD has a thickness of about 1.2 mm and that of a DVD has a thickness of about 0.6 mm, in order to discriminate only a CD and a DVD from each other, by setting the threshold value to, for example, 1.0 mm, the optical medium is determined as a DVD when an image is formed on the signal surface at a distance of less than the threshold value from the surface, and as a CD when an image is not formed at the above-mentioned distance.
However, since the protective layer of a dual disc has a thickness of 0.9 mm, in order to handle the optical disc medium, the threshold value for a CD must be set to a value between 0.9 mm and 1.2 mm and the threshold value for a DVD must be set to a value between 0.6 mm and 0.9 mm.
In general, a feed rate of an actuator for moving the objective lens of the optical pickup relative to the surface of the optical disc may change according to environmental factors such as temperature. In consideration of the change in feed rate, it is not realistic to set the threshold value to a value of around 0.3 mm. Thus, depending on the type of optical disc, it is difficult to discriminate the type of a DVD, a CD, or a dual disc under the current circumstances.